NOTE Aurioles-Gamboa et at: Mass strandings of Pleuroncodes planipes 



469 



had an overrepresentation of females (1:1.23), sug- 

 gesting that a higher abundance of females in 

 stranded crabs would be a reflection of the sex ratio 

 in the bay. Additional evidence for a female biased 

 sex ratio in coastal waters comes from Serrano ( 1991), 

 who reported higher frequency of females inshore 

 during the breeding season. Similar findings were 

 obtained by Escoto and Orellana (1981) for pelagic 

 red crabs off the Nicaragua coast, and for a closely 

 related species, P. monodon, from Chile (Arana and 

 Culquichicon 4 ). However, several authors have re- 

 ported that the total sex ratio on the continental shelf 

 is male biased (Boyd, 1962; Boyd and Johnson, 1963; 

 Serrano, 1991). Boyd (1962), suggested that the sex- 

 ratio differences on the continental shelf could be due 

 to one or a combination of the following causes: 1) a 

 deviation in primary sex ratio, 2) lower survival rate 

 of females, or 3) the fact that plankton nets may not 

 sample males and females with equal effectiveness. 

 The hypothesis that females die at a higher rate 

 than males is supported by the present data, since 

 females were more abundant where there was a natu- 

 ral cause of mass mortality. It has been mentioned 

 that the breeding season of the species is synchro- 

 nized to the time (winter-spring) when coastal up- 

 welling is more intense. Pelagic red crabs may be 

 more abundant in places where phytoplankton is 

 more plentiful (Blackburn, 1969). Thus, it can be 

 advantageous for pelagic red crab females to move 

 to shore and release larvae in upwelled water. How- 

 ever, by doing so, females are more likely to enter 

 Bahia Magdalena or other coastal lagoons and die in 

 accidental strandings. 



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